1. Field of the Invention
The present invention relates to a process for the preparation of ethylene-based copolymers and, more particulary, it relates to a slurry process for the preparation of elastomeric copolymers of ethylene.
2. Description of the Prior Art
Among the ethylene-based elastomeric copolymers, only ethylene-propylene (EPM) and ethylene-propylene-diene (EPDM) elastomers are produced on a commercial scale, at the date of the present invention.
The industrial production of EPM and EPDM elastomers is currently carried out in the presence of Ziegler-Natta vanadium-based catalysts, by solution or slurry processes.
In the solution processes the comonomers are dissolved in a solvent, generally hexane, in which the formed polymer is soluble. In the slurry processes the reaction medium is essentially constituted by liquid olefins and the polymer is formed as a precipitate suspended in the liquid phase.
A slurry process offers a number of advantages over a solution process, namely:
no stirring viscosity problems; PA1 very homogeneous reaction medium; PA1 easier removal of the reaction heat; PA1 increased reactor throughput owing to higher concentration of the polymer in the medium; PA1 higher polymerization yields; PA1 capability of producing very high MW polymers; PA1 energy savings for the recovery of the polymer; PA1 lower investment and production costs. PA1 (A) a metallocene compound of formula (II) EQU (C.sub.5 R.sup.1.sub.5-m)R.sup.2.sub.m (C.sub.5 R.sup.1.sub.5-m) MQ.sub.2 (II) PA1 wherein PA1 M is Ti, Zr, Hf or V; the C.sub.5 R.sup.1.sub.5-m groups, same or different, are cyclopentadienyl rings equally or differently substituted; PA1 R.sup.1, same or different, are hydrogen atoms or alkyl, alkenyl, aryl, alkylaryl or arylalkyl radicals containing from 1 to 20 carbon atoms, which can also contain Si or Ge atoms, or Si(CH.sub.3).sub.3 groups, or also two or four substituted R.sup.1 of a same cyclopentadienyl group can form one or two rings having from 4 to 6 carbon atoms; PA1 R.sup.2 is a group bridging the two cyclopentadienyl rings and is selected from CR.sup.3.sub.2, C.sub.2 R.sup.3.sub.4. SiR.sup.3.sub.2, Si.sub.2 R.sup.3.sub.4, GeR.sup.3.sub.2, Ge.sub.2 R.sup.3.sub.4, R.sup.3.sub.2 SiCR.sup.3.sub.2, NR.sup.1 and PR.sup.1, wherein R.sup.3, same or different, are defined as R.sup.1 or two or four substituents R.sup.3 can form one or two rings having from 3 to 6 carbon atoms; PA1 Q, same or different, are halogen atoms, hydrogen atoms, R.sup.1, OR.sup.1, SR.sup.1, NR.sup.1.sub.2 or PR.sup.1.sub.2; PA1 m can be 0 or 1; optionally pre-reacted with an organometallic compound of aluminum of formula (III): EQU AlR.sup.4.sub.3-z H.sub.z (III) PA1 wherein R.sup.4, same or different, are alkyl, alkenyl or alkylaryl radicals containing from 1 to 10 carbon atoms, and z can be 0 or 1; and PA1 (B) an alumoxane, optionally in admixture with an organometallic compound of aluminum of formula (III): EQU AlR.sup.4.sub.3-z H.sub.z (III) PA1 wherein R.sup.4 and z are defined as above, or one or more compounds able to give a metallocene alkyl cation. PA1 (A) ethylene-bis(4,5,6,7-tetrahydroindenyl)zirconium dichloride, and PA1 (B) a compound selected from tetraisobutyl-dialumoxane (TIBAO) and the product of the reaction between aluminum triisobutyl (TIBAL) and water. PA1 product of reactivity ratios r.sub.e.multidot.r.sub..alpha. lower than 1 and, preferably, lower than 0.8; PA1 less than 2%, preferably less than 1%, of CH.sub.2 groups in the chain contained in sequences (CH.sub.2).sub.n, wherein n is an even integer; PA1 intrinsic viscosity higher than 0.2 dl/g. PA1 M. Kakugo et al., Macromolecules 15, 1150-1152 (1982); PA1 L. Sun, S. Lin, J. Polym. Sci.- Part A-Polym. Chem. 28, 1237, (1990); PA1 E.T. Hsieh, J. C. Randall, Macromolecules 15, 353 (1983); PA1 H.N. Cheng, J. Polym. Phys. 21, 573, (1983). PA1 polyenes able to give unsaturated units, such as:
However, a major problem of a slurry process arises from the adhesive properties of the rubbery material. As a matter of fact, the solid particles of the polymer have a tendency to stick to one another or to the wall surface and to the agitating element of the reactor. This worsens to a large extent the diffusion of ethylene in the reaction medium and, what is more, causes intensive fouling of the reactor, thus rendering the preparation of the polymer very difficult.
In order to avoid such problems, a solvent, such as toluene or cyclohexane, can be added to the reaction medium, which acts both as antifouling agent and as vehicle of the catalyst system. The use of a low boiling diluent, such as propane, has also been proposed. As a result, however, the above indicated advantages of a slurry process are drastically decreased.
Another solution which has been proposed to render the process in bulk possible, is the addition of antistatic agents into the polymerization reactor. This solution, however, is not completely satisfactory and, moreover, has the drawback of introducing undesired compounds in the final product.
Recently, processes have been disclosed for the preparation of elastomeric ethylene-based copolymers in the presence of metallocene/alumoxane catalysts.
European patent application No. 347,128 discloses a process for producing an ethylene/.alpha.-olefin elastomer in slurry polymerization, utilizing a zirconocene/alumoxane catalyst supported on a silica gel support. The examples relate to the preparation of ethylene/propylene copolymers in liquid propylene. It is said that, unless the supported catalyst is prepolymerized with ethylene or another .alpha.-olefin before being used in the slurry polymerization process, the reactor fouling invariably occurs to a very large extent.
In European patent application No. 535,230, a slurry polymerization process for preparing an ethylene-based copolymer has been proposed, which prevents the occurence of fouling. This process is carried out in the presence of both a polysiloxane additive and a silica gel supported zirconocene/methylalumoxane catalyst. All of the examples relate to ethylene/propylene elastomers. In the comparative examples in which no polysiloxane additive has been used, clogging and jamming have been observed.
In International patent application PCT/EP93/01528, there is described a process for the preparation of ethylene/1-butene or ethylene/1-butene/diene elastomeric copolymers in the presence of a metallocene catalyst, wherein the reaction medium is substantially constituted of liquid 1-butene. This process is free of fouling phenomena of the reactor.